1. Field of the Invention
The present invention relates to an osteogenic interbody fusion implant device and, more particularly, to a non-threaded intervertebral bone implant having a stepped configuration which facilitates securement of the implant within the intervertebral site.
2. Description of the Related Art
The spine is a flexible column formed of a series of bone called vertebrae. The vertebrae are hollow and piled one upon the other, forming a strong hollow column for support of the cranium and trunk. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected together by means of articular processes and intervertebral, fibro-cartilagineous spaces.
The intervertebral fibro-cartilages are also known as intervertebral disks and are made of a fibrous ring filled with pulpy material. The disks function as spinal shock absorbers and also cooperate with synovial joints to facilitate movement and maintain flexibility of the spine. When one or more disks degenerate through accident or disease, nerves passing near the affected area may be compressed and are consequently irritated. The result may be chronic and/or debilitating back pain. Various methods and apparatus, both surgical and non-surgical, have been designed to relieve such back pain.
One method, interbody fusion, involves stretching the spine into a natural position so that nerve root canal sizes are increased and nerve irritation is eliminated or reduced. The space between vertebrae is maintained by fusing the vertebrae in the affected area together at a fixed distance. Numerous prosthetic implants have been suggested to fill the void between vertebrae. For example, U.S. Pat. No. 4,936,848 describes a spherical cage implant made of metal or ceramics which is inserted between adjacent vertebrae. The cage has an interior cavity within which bone fragments are inserted. Such bone fragments may be autogenic and are intended to promote subsequent bone growth and fusion of the vertebrae.
Another method of preventing contact of vertebrae is described in U.S. Pat. No. 5,011,484 wherein a stud-shaped insert is inserted longitudinally between two vertebrae and held there by a retainer. U.S. Pat. No. 4,309,777 describes an artificial intervertebral disc having upper and lower discs which are connected to each other by springs. The artificial disc is held in between vertebrae by spikes which project from the disc into the vertebrae. U.S. Pat. No. 4,743,256 describes a rigid, porous plug which can be inserted between vertebrae and held in place by prongs or screws. The porous nature of the plug is alleged to facilitate ingrowth of bone tissue.
An implant bone plug for insertion between vertebrae is also described in U.S. Pat. No. 4,878,915 wherein, in one embodiment, the exterior of the plug is provided with external threading which will, when the plug is rotated, advance the plug into prepared sites between the vertebrae. A portion of the plug is provided with a slot designed to receive the end of a key which is used to rotate the plug. U.S. Pat. No. 5,105,255 describes a method for forming a bore between two adjacent vertebrae and insertion of graft medium such as finely chopped cortical or cancellous bone chips. U.S. Pat. No. 4,961,740 is directed to a substantially open fusion cage which is inserted between adjacent bony surfaces between vertebrae by screwing the cage into place. The cage may be filled with bone chips or other bone inducing substances and, when inserted into the intervertebral space, immediate contact between the bone inducing substance contained within the cage and the native bone occurs through the outer surface of the cage.
Ideally, a fusion graft should stabilize the intervertebral space and become fused to adjacent vertebrae. Moreover, during the time it takes for fusion to occur, the graft should have enough structural integrity to withstand the stress of maintaining the space without substantially degrading or deforming and have sufficient stability to remain securely in place prior to actual bone ingrowth fusion. Consequently, a fusion graft should contain some kind of anchor and, additionally, a bone inducing substance which causes rapid bone growth and quick fusion of the graft to adjacent vertebrae. Furthermore, the material from which the fusion graft is made should be biocompatible and closely mimic the body's naturally occurring tissues.
All the above-described implants are intended to support and maintain an appropriate intervertebral space. Unfortunately, those implants may not fit certain ideal criteria for an interbody fusion graft. For example, many of the implants such as the one described in U.S. Pat. No. 4,936,848 are made of metals and ceramics and, while biocompatible, do not precisely mimic the body's natural bone tissue. U.S. Pat. No. 5,015,255 describes a graft in the form of bone chips which may eventually result in fusion between the vertebrae. If adequate fusion of the bone chips occurs, the final fused graft may closely mimic the body's naturally occurring tissues. However, when the bone chips are inserted, they are unconfined and may not remain contained between the vertebrae for a sufficient time to adequately fuse to each other and to adjacent vertebrae. The bone plug disclosed in U.S. Pat. No. 4,878,915 has a threaded outer surface to assist in placement of the implant between the adjacent vertebrae. The external threads, however, compromise the strength of the implant. In addition, the threaded bone implant may have a tendency of backing out of the prepared bore.
Consequently, there is a need for improved interbody fusion implants which more closely adhere to the ideal of a spinal fusion implant.